Auto merge of #46808 - eddyb:issue-46769-quick, r=arielb1

rustc: ensure optimized enums have a properly aligned size.

Fixes #46769 by padding the optimized enums wrapping packed data as necessary.

Note that this is not the only way to solve this - on nightly, #46436 makes it easier to fix without adding new padding because of the replacement of `packed` flags with a non-redundant scheme.
But because it can't be backported, the optimal fix will be in a separate nightly-only PR (#46809).

src/librustc/ty/layout.rs

@@ -1492,7 +1492,7 @@ impl<'a, 'tcx> LayoutDetails {
                             }).collect::<Result<Vec<_>, _>>()?;
 
                             let offset = st[i].fields.offset(field_index) + offset;
-                            let LayoutDetails { size, mut align, .. } = st[i];
+                            let LayoutDetails { mut size, mut align, .. } = st[i];
 
                             let mut niche_align = niche.value.align(dl);
                             let abi = if offset.bytes() == 0 && niche.value.size(dl) == size {
@@ -1504,6 +1504,7 @@ impl<'a, 'tcx> LayoutDetails {
                                 Abi::Aggregate { sized: true }
                             };
                             align = align.max(niche_align);
+                            size = size.abi_align(align);
 
                             return Ok(tcx.intern_layout(LayoutDetails {
                                 variants: Variants::NicheFilling {

src/test/run-pass/packed-struct-optimized-enum.rs

@@ -16,14 +16,21 @@ impl<T: Copy> Clone for Packed<T> {
     fn clone(&self) -> Self { *self }
 }
 
-fn main() {
-    let one = (Some(Packed((&(), 0))), true);
+fn sanity_check_size<T: Copy>(one: T) {
     let two = [one, one];
     let stride = (&two[1] as *const _ as usize) - (&two[0] as *const _ as usize);
+    assert_eq!(stride, std::mem::size_of_val(&one));
+}
 
+fn main() {
     // This can fail if rustc and LLVM disagree on the size of a type.
     // In this case, `Option<Packed<(&(), u32)>>` was erronously not
     // marked as packed despite needing alignment `1` and containing
     // its `&()` discriminant, which has alignment larger than `1`.
-    assert_eq!(stride, std::mem::size_of_val(&one));
+    sanity_check_size((Some(Packed((&(), 0))), true));
+
+    // In #46769, `Option<(Packed<&()>, bool)>` was found to have
+    // pointer alignment, without actually being aligned in size.
+    // E.g. on 64-bit platforms, it had alignment `8` but size `9`.
+    sanity_check_size(Some((Packed(&()), true)));
 }